Experimental results and discussions

Results of tensile tests are presented in Fig. 2-12 to Fig. 2-20. Typical stress-strain curves for conventional CFRP laminates and various UACS laminates with Lx = 5 mm are presented in Fig. 2-12. It is seen that the conventional laminate without slits shows a nonlinear behavior near the peak value and that newly designed UACS laminates also show a similar nonlinear behavior near their peak values although the nonlinear range is relatively smaller than that of the conventional laminate. This nonlinear behavior is considered to reflect the damage progress of matrix cracking and delamination before fiber completely breaking of 0° plies, which can be seen in the analysis of fractured specimens later. In contrast, in the case of the existing UACS laminate with continuous slits, stress increases linearly with the increase of strain until failure. This fact means that the final failure of the laminate happens instantaneously with the delamination progressing in the existing UACS laminate with continuous slits. Besides, newly

Figure 2-11: MTS 810 material-testing system used in the tensile tests of UACS laminates.

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designed UACS laminates appear to have almost the same tensile modulus as a conventional laminate without slits, which is a little higher than that of the UACS laminate with continuous slits. The reason for this feature is that the in-plane shear stiffness of a UACS ply with continuous angled slits is lower than those of newly designed UACS plies with discontinuous angled slits, which finally lead to a lower tensile modulus of the UACS laminate with continuous angled slits. The tensile strength values of both kinds of newly designed UACS laminates are also higher than the existing UACS laminate with continuous slits.

Detailed values of the tensile modulus and strength are given in Fig. 13 and Fig. 2-14. Each value in the figure represents the average over four specimens and the error bar indicates the scatter range of test results. Comparing with a laminate without slits, the modulus and strength of UACS laminate with continuous slits reduces by 4.0% and 42.2%, the laminate with staggered slits reduces by 2.6% and 36.5%, and the laminate with bi-angled slits reduces by 0.3% and 33.7%, respectively. Comparing with an existing UACS laminate with continuous slits, newly designed UACS laminates with

bi-Figure 2-12: Typical stress-strain curves of cured UACS laminate with various slit patterns.

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angled slits and with staggered slits enhance the tensile strength by 14.7% and 9.9%, respectively. Evidently, out of the three types of laminates studied, the UACS laminate with bi-angled slits gives the best tensile properties among the UACS laminates. It should be noted that in previous research [1, 3] the UACS laminate is cured by hot pressing at 2 MPa pressure, which is much higher than the present curing pressure of 0.3 MPa, so that the thickness of each ply of the UACS laminate changes from original 0.14 mm to 0.1 mm. Hence, the tensile strength of the UACS laminate with continuous 11.3-degree angled slits arrives at almost 80% of that of the conventional laminate without slits and with each ply being 0.14 mm in thickness in [3] because of the significant influence of ply thickness on the tensile strength of UACS laminates. In present paper, all laminates are cured by autoclave at 0.3 MPa pressure and each ply of all laminates is almost the same at about 0.2 mm in thickness. Therefore, the present test results do not contain the influence of ply thickness and only reveal the influence of slit patterns on the tensile properties.

Figure 2-13: Tensile modulus of various quasi-isotropic laminates.

50.26

48.26 48.95 50.10

0 10 20 30 40 50 60 70

Various quasi-isotropic laminates

Modulus E11 (GPa) .

No slit Continuous slit

Staggered slit: S(5) Bi-angeld slit: B(5)

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Typical images of fractured specimens are shown in Fig. 2-15. In the cases of conventional laminate without slits the breakage of 0° plies dominates the final failure. In newly designed UACS laminates with staggered and bi-angled slit patterns, it is also observed that a few fibers of 0° plies are broken. On the other hand, relatively less fiber breakages are observed in the case of UACS laminate with continuous slits, and the portion of delamination is a little bit higher than the cases of newly designed UACS laminates. These results reveal that effective utilization of the fiber strength of 0° plies can enhance the strength of the laminate. These fiber breakages are supposed to be related to the nonlinear regions of stress-strain curves in Fig. 2-12.

Figure 2-14: Tensile strength of various quasi-isotropic laminates.

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(a) No slit (b) Continuous slit pattern

(c) Staggered slit pattern (d) Bi-angled slit pattern

To further understand the failure mechanisms of the three types of UACS laminates, the cross-sections of specimens loaded at the load levels of 90% and 95% of the strength are polished and then observed using optical microscopy. Fig. 2-16 shows the polished cross-section images of continuous slit pattern UACS laminate at the load level of 90% of the strength. Damage images in other patterns of UACS laminates display the similar properties. Matrix cracking in the neighboring two 90° plies (8th and 9th) occurs as the initial damage in the whole of laminates to all patterns of UACS. However, this form of damage mode is not the critical damage mode resulting to the final fracture and not

Figure 2-15: Typical images of fractured specimens of various quasi-isotropic laminates.

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develops to the adjacent plies. The other damage mode observed under this load level is the slit cracking in 0° plies. Crack easily occurs in the slit regions in 0° plies under longitudinal load due that the fibers are cut off in these regions which are filled with resin after cure. Subsequently, significant concentration of shear stresses occurs around the cracked slits between 0° plies and its adjacent plies, which turns to lead to the initiation of the delamination along the interlaminate between 0° plies and its adjacent plies, as shown in Fig. 2-16.

At the load level of 95%, various damage morphologies are observed, as shown in Fig.

2-17. Relatively more matrix cracks are observed in laminates with two new slit patterns compared to the laminate with continuous slits because the former laminates carry higher load than the latter. On the other hand, relatively large delamination extension from the cracked slit of the 0° ply is observed in the laminate with continuous slits compared to laminates with two new slit patterns. In addition, it is interesting that no crack occurs within the slits of 90° and ±45° plies.

Figure 2-16: Cross-section images of continuous slit pattern UACS laminate at the load level of 90% of the laminate strength.

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(a) Continuous slit pattern

(b) Staggered slit pattern

(c) Bi-angled slit pattern

Based on the observed results of fractured specimens and specimens loaded at 90%

and 95% of the laminate strength, basic damage progress behaviors are schematically depicted in Fig. 2-18. Firstly, 90° plies, 45° plies, and slits in 0° plies crack and then Figure 2-17: Cross-section images of various UACS laminates after tensile test at the load level

of 95% of the laminate strength.

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delamination occurs and progresses between 0° ply and adjacent ±45° plies. Finally, the delamination adjoining matrix cracks leads to the fracture of the specimen. The main difference between UACS laminate with continuous slits and two newly designed UACS laminates is that large delamination develops in the laminate with continuous slits as indicated in the figure by a yellow line and arrows. This schematic image is consistent with Fig. 2-15(b) where many fragments of 0° plies are pulled out due to large delamination. In contrast, delamination occurred in laminates with two new slit patterns stops progressing when it meets the matrix crack so that 0° plies can still carry higher load.

Fig. 2-19 presents the distribution schematic of slits in 0° plies and adjacent 45° plies of the three kinds of UACS laminates and the gray and blue lines strand for slits in 0°

plies and 45° plies respectively. According to the damage progress analysis under the load level of 95% final load, the delamination develops along slit direction of 0° plies after occurring in the crossing points of slits. To continuous slit pattern UACS, distribution of slits is the most regular and delamination develops most rapidly and

Figure 2-18: Schematic of damage progress for UACS laminates under tension.

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delamination areas are linked together easily. To staggered and bi-angled slit patterns, delamination is inhibited at the tips of discontinuous slits. The delamination development behavior is the main difference of damage progress in UACS laminates with different patterns slit distributions.

(a) Continuous slit pattern

(b) Staggered slit pattern

(c) Bi-angled slit pattern

In conclusion, delamination extension in UACS laminates is quite complicated as indicated in [1, 3]. Further study combined with numerical analysis is necessary to reveal

Figure 2-19: Slits distributions of 0° plies and adjacent 45° plies in various UACS laminates.

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the failure mechanism of the three kinds of UACS laminates, which will be clarified in the next chapter.

Results associated with the influence of slit length on the tensile properties of laminates with two newly designed slit patterns are presented in Fig. 2-20. Tensile modulus seems to have slight variation, but tensile strength appears to obviously decrease with the increase of slit length.

(a) Tensile modulus

(b) Tensile strength

49.0 49.8 50.0 49.6 50.1 49.4 49.4 50.5

0 10 20 30 40 50 60 70

Various UACS quasi-isotropic laminates Modulus E11 (GPa)

S(5) S(7.5) S(10) S(12.5) B(5) B(7.5) B(10) B(12.5)

Figure 2-20: Influence of slit length on the tensile properties of laminates with two new slit patterns.

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